Articles

    It's Glass-Time

    Kids, what does the word "glass" make you think of? Glass objects can come in all shapes, sizes, and colors. List all the objects you come across in one day that are made out of glass. Where does glass come from? One example is window glass, also called "soda lime" glass. It is made mostly from a pure, white sand called silica. Also added are soda (soda ash or sodium carbonate) and lime (limestone or calcium carbonate). Soda ash makes the sand melt more easily and lime makes the glass hard and waterproof. In a glass factory furnace, the mixture is heated to 2,500°F for up to a day. Molten glass is viscous and small bubbles take a long time to disappear. To visualize this, mix powdered sugar in a glass of corn syrup and watch how long it takes for the bubbles to rise.

    What different kinds of glass are there? Huge windows are called float glass because of the way they are made. Since they need to be perfectly flat and smooth, the molten glass is floated on a layer of molten metal (often zinc) and is then cooled very slowly. To visualize this, pour some cooking oil over the back of a spoon onto water in a clear bowl. See how the oil layer floats on top? Some other types of glass are made from other ingredients added to the silica. "Pyrex" cookware is borosilicate, made with some boron to withstand quick temperature changes. Lead crystal incorporates lead to sparkle and make the glass easy to cut and engrave.

    What do you know about glass recycling? Amber and green glass are separated because a coloring agent has been added that cannot be removed. Therefore, brown bottles can only make other brown bottles. Recycled glass is crushed into pieces called cullet. Cullet, which melts at a lower temperature, is mixed with the raw materials silica, soda, and lime. This process reduces air pollution by 20%, water pollution by 50%, and saves space in landfills. However, only about 10% of the glass used in the U.S. is recycled.

    --------------

    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    December 1996

    ----------------

    References: "Glass" by Jane Chandler and the "Newton's Apple" web page: www.ktca.org/newtons.